Electron Beam Current Density Research Articles

Simulation study of a novel small animal FLASH irradiator (SAFI) with integrated inverse-geometry CT based on circularly distributed kV X-ray sources

Ultra-high dose rate (UHDR) radiotherapy (RT) or FLASH-RT can potentially reduce normal tissue toxicity. A small animal irradiator that can deliver FLASH-RT treatments similar to clinical RT treatments is needed for pre-clinical studies of FLASH-RT. We designed and simulated a novel small animal FLASH irradiator (SAFI) based on distributed x-ray source technology. The SAFI system comprises a distributed x-ray source with 51 focal spots equally distributed on a 20 cm diameter ring, which are used for both FLASH-RT and onboard micro-CT imaging. Monte Carlo simulation was performed to estimate the dosimetric characteristics of the SAFI treatment beams. The maximum dose rate, which is limited by the power density of the tungsten target, was estimated based on finite-element analysis (FEA). The maximum DC electron beam current density is 2.6 mA/mm2, limited by the tungsten target's linear focal spot power density. At 160 kVp, 51 focal spots, each with a dimension of 2times 20 mm2 and 10° anode angle, can produce up to 120 Gy/s maximum DC irradiation at the center of a cylindrical water phantom. We further demonstrate forward and inverse FLASH-RT planning, as well as inverse-geometry micro-CT with circular source array imaging via numerical simulations.

MODELING AND PRELIMINARY EXPERIMENTAL RESULTS OF THIN FILM HEATING DURING THE PASSAGE OF A HIGH-ENERGY ELECTRON BEAM

The preliminary experimental results as well as modeling the heating of thin-foil materials during the passage of high-energy electrons with energy of 15 MeV are presented. Foils of 50 μm titanium, 50 μm aluminum and 125 μm Kapton® were chosen as the test targets. A calculation technique has been developed, which consists of automating the finite difference method applying Python programming language tools. These tools allowed solving the problem of heat distribution in the thin foil, taking into account the ionization losses of the primary electron beam and the black body radiation. The data on the surface temperature distribution of the research samples were obtained. The time for establishing thermal equilibrium was determined taking into account the distribution of the electron beam current density. It is shown that optimization of the main parameters of the high-energy electron beam beam (for example the current density) makes it possible to neglect the thermal loads on these films, which was confirmed during bench tests at 30 MeV electron accelerator of the IHEPNP NSC KIPT.

Effect of q-Nonextensively Distributed Plasma Electrons on Double Sheath Characteristics and Virtual Cathode Formation Associated With Electron Emitting Surfaces

A double sheath characteristics and virtual cathode formation in a plasma containing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$q$</tex-math> </inline-formula> -nonextensive distributed electrons, thermally emitted or secondary emitted electrons from the cathode wall, and ions are studied numerically. The distribution of primary (plasma) electrons is taken to be nonextensive, as per Tsallis statistics. The electric field at the cathode, the ratio of ion to electron current density and the velocity of ions are evaluated and the results are compared with the case of electrons having Boltzmann distribution. The variation of the electric potential, space charge density, and sheath thickness are analyzed for the cases of different nonextensivity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$q$</tex-math> </inline-formula> . The electric field at the cathode becomes zero at a lower value of electron beam current density, and hence the formation of the virtual cathode takes place earlier for the case of super-extensive distributed electrons as compared to that of the Boltzmann-distributed electrons. For super extensively distributed electrons, the electric potential drops slowly to a minimum, the velocity of positive ions increases inside the sheath, and an increase in the sheath thickness is observed.

Electrical Characterization of Plasma-Assisted Electron Beam Source With Sub-mm Sheet Aperture

In this article, the electrical characterization of a plasma-assisted electron beam source with a sub-mm sheet aperture has been performed. The developed sheet electron beam source comprises a hollow cathode cavity, two floating anodes, and one ground potential anode with a sheet aperture of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\times0.3$ </tex-math></inline-formula> mm. The source has been characterized under varying applied gap potential range, i.e., 10–30 kV in self-breakdown operating mode. The breakdown parameters and resultant beam parameters, such as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V} - {I}$ </tex-math></inline-formula> characteristics, electron beam current density, duration to reach the conductive phase, and the energy transformation efficiency of the developed source, have been analyzed. The successive breakdown phenomenon has also been presented in the profile of the developed electron beam source.

Shot-to-shot two-dimensional photon intensity diagnostics within megahertz pulse-trains at the European XFEL.

Characterizing the properties of X-ray free-electron laser (XFEL) sources is a critical step for optimization of performance and experiment planning. The recent availability of MHz XFELs has opened up a range of new opportunities for novel experiments but also highlighted the need for systematic measurements of the source properties. Here, MHz-enabled beam imaging diagnostics developed for the SPB/SFX instrument at the European XFEL are exploited to measure the shot-to-shot intensity statistics of X-ray pulses. The ability to record pulse-integrated two-dimensional transverse intensity measurements at multiple planes along an XFEL beamline at MHz rates yields an improved understanding of the shot-to-shot photon beam intensity variations. These variations can play a critical role, for example, in determining the outcome of single-particle imaging experiments and other experiments that are sensitive to the transverse profile of the incident beam. It is observed that shot-to-shot variations in the statistical properties of a recorded ensemble of radiant intensity distributions are sensitive to changes in electron beam current density. These changes typically occur during pulse-distribution to the instrument and are currently not accounted for by the existing suite of imaging diagnostics. Modulations of the electron beam orbit in the accelerator are observed to induce a time-dependence in the statistics of individual pulses - this is demonstrated by applying radio-frequency trajectory tilts to electron bunch-trains delivered to the instrument. We discuss how these modifications of the beam trajectory might be used to modify the statistical properties of the source and potential future applications.

Electron gun producing beams with controllable current density

The existing Brillouin-type electron gun at the TwinEBIS test bench is, according to Herrmann theory, capable of producing an electron beam with a current density of 3850 A/cm2 in the 2 T solenoid. To control the electron beam current density and the magnetic flux inside the beam, the existing electron gun - now using purely electrostatic focusing - can be modified by permitting magnetic flux to reach the cathode. In such a configuration, the stabilizing magnetic flux inside the electron beam can be controlled by changing the current in the magnet coil surrounding the cathode. The radial oscillations of the electron beam, resulting from the increased magnetic field on the cathode, can be significantly reduced by employing a non-adiabatic magnetic field near the electron gun. This method has been recently developed and successfully used at REXEBIS at CERN. We present the computer simulations of such electro-optical system.

Estimation of point defects content in bulk GaN

The paper reveals the investigation of cathodoluminescent properties of point defects in bulk GaN sample grown by the HVPE technique. The cathodoluminescence spectra of GaN exhibit two broad luminescence bands in the blue and yellow optical ranges. The studies have shown that each of the observed luminescence bands is associated with the emission of several point defects - luminescence centers with similar spectral positions, but different decay times. The paper proposes a technique for estimation of the luminescent centers relative content. The technique is based on measuring the cathodoluminescence intensity dependence on the electron beam current density and decay times of the bands. The changes in the relative contents of point defects - luminescent centers are determined for the back and front sample regions – at the beginning of growth and near the sample surface. The excitation capture efficiency was determined for luminescence centers with the longest decay time emitting in the yellow range. It was found to be constant for different regions of the sample.

Controllable fabrication of hollow In2O3 nanoparticles by electron beam irradiation.

A growth strategy is presented for controllable fabrication of hollow In2O3 nanoparticles (NPs) via oxidation of In nanocrystals under electron beam irradiation. The morphology of the NPs can be tailored by changing the electron beam energy and current density. Yolk-shell NPs are preferentially formed under 200 keV electron beam irradiation, while hollow NPs are preferentially formed at 300 keV. This work confirms that electron beam irradiation is a valuable method for the engineering and modification of nanomaterials.

Modeling intense-electron-beam generated plasmas using a rigid-beam approximation

A model of an electron-beam–plasma system is introduced to model the electrical breakdown physics of low-pressure nitrogen irradiated by an intense pulsed electron beam. The rapidly rising beam current induces an electric field, which drives a return current in the plasma. The rigid-beam model is a reduction of the problem geometry to cylindrical coordinates and simplifications to Maxwell's equations that are driven by a prescribed electron beam current density. The model is convenient for comparing various reductions of the plasma dynamics and plasma chemistry while maintaining a good approximation to the overall magnitude of the beam-created electric field. The usefulness of this model is demonstrated by coupling the rigid-beam model to a fluid plasma model and a simplified nitrogen plasma chemistry. The dynamics of this coupled system are computed for a range of background gas pressures, and the results are compared with experimental measurements. At pressures 1 Torr and above, the simulated line-integrated electron densities are within a factor of two of measurements and show the same trend with pressure as observed in experiment.

Dose measurement in the TEM and STEM

Practical aspects of dosimetry are considered, including the measurement of electron-beam current and current density. Complications that arise in the case of a focused probe or a STEM image are discussed and solutions proposed. Advantages of expressing the radiation dose in Grays are listed and a simple formula given for converting electron fluence to Gray units, based on a near constancy of the stopping power per atomic electron. Comparisons with stopping-power calculations and EELS measurements suggest that this formula is accurate to within 5%. Based on the stopping power formula, a new way of measuring the local mass-thickness of light-element specimens is proposed. The average energy loss per inelastic collision is shown to be higher than previous expectations.

Direct Visualization of the Earliest Stages of Crystallization.

Investigating the earliest stages of crystallization requires the transmission electron microscope (TEM) and is particularly challenging for materials which can be affected by the electron beam. Typically, when imaging at magnifications high enough to observe local crystallinity, the electron beam's current density must be high to produce adequate image contrast. Yet, minimizing the electron dose is necessary to reduce the changes caused by the beam. With the advent of a sensitive, high-speed, direct-detection camera for a TEM that is corrected for spherical aberration, it is possible to probe the early stages of crystallization at the atomic scale. High-quality images with low contrast can now be analyzed using new computing methods. In the present paper, this approach is illustrated for crystallization in a Ge2Sb2Te5 (GST-225) phase-change material which can undergo particularly rapid phase transformations and is sensitive to the electron beam. A thin (20 nm) film of GST-225 has been directly imaged in the TEM and the low-dose images processed using Python scripting to extract details of the nanoscale nuclei. Quantitative analysis of the processed images in a video sequence also allows the growth of such nuclei to be followed.

Toward Next Generation Plasmonic Nanopore Slit Platform with a ~10 nm Slit-Width

We fabricated various nanoaperture plasmonic platforms for single-molecule detection. We fabricated nanoapertures like nanopores on a pyramid and nanoslits on an Au flat membrane using a Ga ion focused ion beam drilling technique, followed by irradiating with a high energy electron beam, dependent on the electron beam current density to obtain nanoapertures with a few nanometer sizes (circular nanopore, nanoslit pores). We examined their optical characteristics with varying aperture sizes and sample thicknesses. We obtained broad emission spectra in the visible and infrared region from the (7 x 7) slit array and a sharp, strong infrared emission peak from the Au nanoparticle on the substrate. The fabricated Au platform with ~10 nm nanometer opening can be employed as a single-molecule sensor and an infrared thermal emission device.

Variable beam entrance Faraday cup system for pulsed electron beam current profile characterization.

A high accuracy variable beam entrance Faraday cup (VBEFC) system is designed in this work. The presented VBEFC system is designed for the beam current profile measurement of the transient hollow cathode discharge (THCD) generated pulsed electron beam, which is a new source of high energy flux for metallic material processing. By proper designs of the beam entrance array, fast response electron collector, grounding, and shielding, this VBEFC system is capable of determining the radial profile and its temporal evolution of the THCD generated electron beam. The results of the electron beam current and current density distributions collected at varying radial locations and times under multiple voltages are presented in this paper. The experimental results show that both the amplitude and the current density of the THCD electron beam at a given radius always increased with the increase in the accelerating voltage, which is coincident with the self-focused propagation of the electron beam promoted by the voltage.

The study of composition, structure and cathodoluminescent features of YAG:Eu3+ nanoceramics. Excitation capture efficiency of Eu3+ energy levels

The focus of this research paper is upon cathodoluminescent features of YAG:Eu3+ nanoceramics. The concentration series of samples was synthesized from nano powders using low temperature high pressure sintering technique. The composition of the samples has been studied using electron-probe microanalysis. The structure and the grain size of the samples have been studied by X-ray diffraction, scanning electron microscopy and electron backscattered diffraction. Cathodoluminescent study of the samples has been performed. The concentration dependence of cathodoluminescence intensity and kinetic dependencies of CL bands have been obtained. The excitation capture efficiency of the radiative energy levels was estimated based on cathodoluminescence intensity rise rate dependence on electron beam current density. Energy transfer mechanism between the energy levels will be discussed hereinafter.

Enhancement of charge breeding efficiency for rare isotope beam with the control of magnetic field profile and electron beam energy in EBIS

The magnetic field in the ion trap of electron beam ion source (EBIS) determines the current density of electron beam and thus the depth of trap potential, which affects the charge breeding efficiency to desired charge state. An EBIS charge breeder has been constructed to be used for the Rare Isotope Science Project in Korea. A 6 T superconducting solenoid is used for the trap, and uniform magnetic field is extended with correction coils at the ends. The effect of field uniformity on the electron current density is evaluated using TRAK, and it is shown sizable improvement in the charge breeding efficiency for rare isotope beams can be obtained by elaborate magnetic design. Furthermore, the electron beam energy affects the ionization efficiency. The electron energy is often reduced in the trap for optimal matching with charge striping cross section. However, virtual cathode formation can appear in the process of energy reduction. Maximum beam currents limited by electron energy are studied analytically and by TRAK simulation in the view of improving the charge breeding efficiency.

Dust mitigation technology for lunar exploration utilizing an electron beam

Dust mobilized on the lunar surface due to natural processes and/or human activities can readily stick to spacesuits, optical devices, and mechanical components, for example. This may lead to dust hazards that have been considered as one of the technical challenges for future lunar exploration. Several dust mitigation technologies have been investigated over the past years. Here we present a new method utilizing an electron beam to shed dust off of surfaces. Recent studies on electrostatic dust lofting have shown that the emission and absorption of secondary electrons or photoelectrons inside microcavities forming between dust particles can cause the buildup of substantial negative charges on the surrounding particles. The subsequent repulsive forces between these particles can cause their release from the surface. Fine-sized lunar simulant particles (JSC-1A, <25 μm in diameter) are used in our experiments. The cleaning performance is tested against the electron beam energy and current density, the surface material, as well as thickness of the initial dust layer. It is shown that the overall cleanliness can reach 75–85% on the timescale of ~100 s with the optimized electron beam parameters (~230 eV and minimum current density between 1.5 and 3 μA/cm2), depending on the thickness of the initial dust layer. The maximum cleanliness is found to be similar between a spacesuit sample and a glass surface. Future work will be focused on removal of the last layer of dust particles and an alternative method using ultraviolet (UV) light.

Heating effect of the radiation chemistry of polyhydroxystyrene-type chemically amplified resists

The photomask used in the lithography process has the important role of transferring circuit pattern images onto the photoresist. To meet the demand for increased photomask manufacturing throughput, the current density of electron beam (EB) writers has been increased. EB exposure locally increases the resist temperature on the mask substrate depending on various factors, including current density, shot size, and writing order. Resist sensitivity increases with irradiated resist temperature, a phenomenon known as “heating effect”. In this study, we reported the cause of the temperature-dependent sensitivity increase in a polyhydroxystyrene-type chemically amplified resist. The experimental results, including acid generation efficiency measurements and pulse radiolysis with changing temperature, suggest that the heating effect is mainly caused by the deprotonation efficiency associated with the radical cation of the base polymer.

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements.

Electron probe microanalyzer measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~100 μg g-1 level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimize electron beam current density, a rastered 50 × 100 μm electron beam was adequate and minimized sensitivity to small Al-rich contaminants. Reproducible analyses of eleven SIMS-cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 μg g-1• The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: lmilac, Eagle Station and Springwater. Our focus was on Al in olivine, but our technique could be refined to give accurate electron probe measurements for other contamination-sensitive trace elements. For solar wind, it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio.

Kinetic properties of YAG: Eu3+ emission upon electron beam excitation

Luminescent properties of yttrium-aluminum garnet activated by different amount of europium (YAG:Eu3+) single crystals were studied by local cathodoluminescence (CL) technique. Samples were grown by flux method. CL spectra were obtained in the range from 400 to 800 nm with spectral resolution of 0.1 nm. The rise and decay kinetics of various CL bands were investigated. The concentration dependencies of CL intensity and decay times of transitions were plotted. The concentration quenching for 5D1-7F2 Eu3+ transition was observed at about 3.5 at.% of Eu by Y, and for 5D2-7F1 and 5D3-7F3 transitions – at about 2.5–3 at.% of Eu by Y. For transition 5D0-7F1 no concentration quenching was observed. The high-energy excitation capture efficiency by different energy levels of Eu3+ in YAG was estimated based on the dependence of the CL rise rate of these levels on the electron beam current density. It was found that 5D1 energy level excitation capture efficiency is about three times higher than that of the 5D0, 5D2 and 5D3 energy levels.

Quantitative Mapping of the Charge Density in a Monolayer of MoS2 at Atomic Resolution by Off-Axis Electron Holography.

The electric potential, electric field, and charge density of a monolayer of MoS2 have been quantitatively measured at atomic-scale resolution. This has been performed by off-axis electron holography using a double aberration-corrected transmission electron microscope operated at 80 kV and a low electron beam current density. Using this low dose rate and acceleration voltage, the specimen damage is limited during imaging. In order to improve the sensitivity of the measurement, a series of holograms have been acquired. Instabilities of the microscope such as the drifts of the specimen, biprism, and optical aberrations during the acquisition have been corrected by data processing. Phase images of the MoS2 monolayer have been acquired with a sensitivity of 2π/698 rad associated with a spatial resolution of 2.4 Å. The improvement in the signal-to-noise ratio allows the charge density to be directly calculated from the phase images using Poisson's equation. Density functional theory simulations of the potential and charge density of this MoS2 monolayer were performed for comparison to the experiment. The experimental measurements and simulations are consistent with each other, and notably, the charge density in a sulfur monovacancy (VS) site is shown.